The present invention relates to energization control systems for a motor. More particularly, the present invention pertains to an energization control system for a motor for controlling electric current in a coil of each phase of a switched reluctance type motor (called an SR motor hereinafter) applied, for instance, to electric vehicles.
The operational principle of SR motors in which the present invention is applied is explained in FIG. 10. As shown in FIG. 10, an SR motor 1 includes a hollow cylindrical stator 2 and a cylindrical rotor 3 which is rotatably provided in the stator 2 keeping a predetermined gap with the stator 2. On the inner periphery of the stator 2, six radial poles 2a-2f are formed at equal intervals. On the outer periphery of the rotor 3, four radial poles 3a-3d are formed at equal intervals. When two radial poles of the stator 2 (e.g., 2c, 2f) are opposed to two radial poles of the rotor 3 (e.g., 3b, 3d), two other radial poles 3a, 3c of the rotor 3 are located between radial poles of the stator 2, i.e., 2a, 2b, and 2d, 2e respectively. Each pair of opposing radial poles 2a and 2d, 2b and 2e, and 2c and 2f, shares a common circuit including coils 4a and 4d, 4b and 4e, and 4c and 4f respectively.
As shown in FIG. 10, when electric current I1 is supplied to the coils 4a, 4d, magnetic flux is generated in the poles 2a, 2d of the stator 2, and thus attracts the poles 3a, 3c of the rotor 3. As shown in FIG. 10, when the electric current I2 is supplied to the coils 4b, 4e, the magnetic flux is generated in the poles 2b, 2e of the stator 2, and thus attracts the poles 3d, 3b of the rotor 3. As shown in FIG. 10, when the electric current I3 is supplied to the coils 4c, 4f, the magnetic flux is generated in the poles 2c, 2f of the stator 2, and thus attracts the poles 3c, 3a of the rotor 3. Accordingly, by supplying three-phase electric current I1-I3 to the pairs of coils 4a-4c, 4b-4e and 4c-4f synchronous with the rotation of the rotor 3, the rotor 3 can be driven at a desired rotation number. By ON/OFF operation of a switching element 10, each electric current I1-I3 is switched ON and OFF. Each electric current is supplied by electric voltage from a battery 5.
FIG. 11 shows a switching circuit for energizing the coils of SR motor by chopping control shown in FIG. 10. The switching circuit illustrated is only for one phase. In order to drive the SR motor 1 shown in FIG. 10, three systems of the same switching circuit are provided.
In FIG. 11, the switching circuit includes a first switching element 11, a second switching element 12, a first diode 13, and a second diode 14. The first switching element 11 is connected between one end of a phase coil 15 and a high electric potential line 16 of a power source. The second switching element 12 is connected between the other end of the coil 15 and a low electric potential line 17 of a power source. The first diode 13 is connected between one end of the coil 15 and the lower electric potential line 17. The second diode is connected between the other end of the coil 15 and the high electric potential line 16.
The first diode 13 allows the electric current to flow from the low electric potential line 17 to one end of the coil 15. The second diode 14 allows the electric current to flow from the other end of the coil 15 to the high electric potential line 6. Both the first and the second diodes are flywheel diodes. A Japanese Patent Laid-Open Publication No. H07-274569 discloses a switching circuit of this kind. The switching elements 11, 12 may be, for instance, Insulated Gate Bipolar Transistors (IGBT).
There are five methods for chopping control of the SR motor 1 by the switching circuit, which are Soft Chopping, Hard Chopping, 0V Loop (zero-volt loop), DUTY Chopping, and Three-Step OFF. The Soft Chopping is a drive method for maintaining a target electric current value by switching ON/OFF only the first switching element 11 or the second switching element 12. In the Hard Chopping driving method, a target electric current value is maintained by switching ON/OFF both the first and the second switching elements 11, 12. The 0V Loop is a driving method for utilizing the energy by turning off the first switching element 11 and turning on the second switching element 12 during the condition that the electric current is already flowing. In the DUTY Chopping method, ON/OFF of the first switching element 11 is switched while the second switching element 12 is OFF, thus to utilize the electric current by degrees. The Three-step OFF is a driving method varying the operation from either one of Soft Chopping or Hard Chopping, 0V Loop, and to DUTY Chopping.
FIG. 12 shows a wave form of switching circuit operated by Soft chopping. An upper signal shown as (b) of FIG. 12 corresponds to a drive signal for actuating the switching element 11. A lower signal shown as (c) of FIG. 12 corresponds to a drive signal for driving the switching element 12. The upper signal which repeats switching ON/OFF shown in FIG. 12 is given to a base of the switching element 11. The lower signal which regularly maintains ON shown in FIG. 12 is given to a base of the switching element 12.
When both the upper signal and the lower signal are ON, the switching elements 11, 12 are conductive, and thus the electric current flows from the high electric potential line 16 to the low electric potential line 17 via the switching element 11, the coil 5, and the switching element 12. When the upper signal is switched to OFF, switching element 11 is disconnected. The lower signal maintains ON. In this condition, the second switching element 12 is conducted and the first diode 13 allows the electric current flow according to the accumulated energy in the coil 15b. The current flows from the coil 15 to the low electric potential line 17 via the second switching element 12. Then, when the upper signal is switched to be ON again, the switching element 11 is conductive, and thus the electric current flows from the switching element 11 to the switching element 12 via the coil 15.
By repeating the forgoing operation, electric current shown in FIG. 12 flows in the coil 15. In FIG. 12, rise of ripple is due to the rise of the electric current flowing in the coil 15 by conduction of the switching element 11. Drop of ripple is due to the moderate reduction of the energy accumulated in the coil 15 by disconnection of the switching element 11. The target value of the electric current is determined at a predetermined value in order to obtain a necessary torque in accordance with the driving condition, when the SR motor is applied, for instance, to the electric vehicle.
In the switching circuit shown in FIG. 11, the switching elements 11, 12 develop heat by energization. An IGBT used as the switching elements 11, 12 is destroyed when the temperature is greater than 150xc2x0 C. Thus, a temperature sensor is positioned near the switching elements 11, 12 to restrict the electric current flowing in the coil 15 for preventing a further increase of the temperature when the temperature detected by the temperature sensor is increased, for example, to 120xc2x0 C.-130xc2x0 C.
On one hand, the upper signal explained in FIG. 12 repeats switching ON/OFF alternatively. On the other hand, the lower signal maintains ON condition. Thus, the switching element 11 repeats the switching ON/OFF and the switching element 12 is maintained to be ON. Accordingly, duration of ON period of the switching element 12 becomes longer than that of the switching element 11 and switching number of the switching element 11 becomes greater than that of the switching element 12. Hence, switching loss of the switching element 11 becomes greater, the temperature increase of the switching element, 11 becomes greater than that of the switching element 12, and the heat generation of each switching element becomes unbalanced.
In order to balance the temperature increase of the switching elements 11, 12, Japanese Patent Application Laid-Open Publication No. 2000-270591 by the applicant discloses a control method for switching elements 11, 12 to be ON alternatively by switching a period for maintaining ON of the upper signal and the lower signal at a predetermined time by a chopping switching signal shown in FIG. 12.
The chopping switching signal is switched at a predetermined time following the order from a CPU. In the aforementioned application, the condition maintaining ON and the condition repeating switching ON/OFF of the upper signal and the lower signal were switched immediately following the switching signal. In this condition, every time switching on the chopping side is performed, a loss is generated by one, and the accumulation of the loss thereof deteriorates the operational efficiency of the motor.
As shown in FIG. 12, according to the foregoing application, the time period for being ON/OFF of the upper signal and the lower signal is predetermined so that both the upper signal and the lower signal have a chopping operation for a predetermined time period. However, when the switching is performed only for the predetermined time period, irrespective of the predetermined time period, the level of both the upper signal and the lower signal is switched, and thus the number of ON/OFF is increased by one every time the switching is performed. For example, the lower side may perform ON/OFF ten times contrasted to nine times of ON/OFF at the upper side. This phenomenon is not favorable regarding the balance of heat development.
A need thus exists for an improved energization control system for a motor that obviates drawbacks associated with known energization control systems for a motor described above.
A need also exists for an energization control system for a motor for keeping the balance of heat development of the of two switching elements as equal as possible.
Accordingly, it is an object of the present invention to provide an improved energization control system for a motor which obviates the above drawbacks. It is another object of the present invention to provide an improved energization control system for a motor which can keep a balance of heat development between two switching elements as equal as possible.
To achieve the aforementioned objects the following technical means are provided for the energization control system of the motor of the present invention which includes a plurality of phase coils wound around a corresponding stator of the motor, a first switching element disposed between one end of one of the coils and one side of a power source line and a second switching element disposed between the other end of the coil and the other side of the power source line. The energization control system for the motor supplies an electric current from the power source line to the coil when the first switching element and the second switching element are simultaneously conducted. The first and second switching elements are controlled under a first condition that one of the first or the second switching elements is switched every first predetermined time while the other of the first or second switching elements is conductive. The first and second switching elements are controlled under a second condition that the other of the first or second switching elements is switched said every first predetermined time while said one of the first or second switching elements is conductive. The first condition and the second condition are repeated synchronized to said first predetermined time every predetermined period.